[Mac] Support option-return to open in new foreground tab.

BUG=http://crbug.com/22882
TEST=Type something into the omnibox.  Option-return should open it in a new foreground tab.
TEST=After using Option-return to open something in a new foreground tab, switch back to the original tab and verify that the url in the omnibox has reverted to the original page's url.
TEST=Option-leftclick on a link in a webpage should still prompt you to "save as."
Review URL: http://codereview.chromium.org/218032

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@27127 0039d316-1c4b-4281-b951-d872f2087c98